1. Field of the Invention
The present invention relates to an energy-trapped type piezoelectric-resonance device utilizing a thickness-extensional vibration mode.
2. Description of the Related Art
An energy-trapped type piezoelectric resonator of a thickness-extensional vibration mode using a piezoelectric ceramic material has been conventionally known. FIGS. 5 and 6 show a conventional piezoelectric resonator, where FIG. 5 is a plan view, and FIG. 6 is a cross sectional view taken along a line A--A shown in FIG. 5. Referring to FIGS. 5 and 6, excitation electrodes 2 and 3 are provided in center parts of major surfaces on both sides of a piezoelectric ceramic body 1. Connecting conductive portions 4 and 5 extending toward side edges of the piezoelectric ceramic body 1 are respectively connected to the excitation electrodes 2 and 3. The piezoelectric ceramic body 1 is polarized in one of the directions of thickness as a whole, as shown in FIG. 6.
As such a piezoelectric resonator, a piezoelectric resonator which can be used in a high frequency band of, for example, 10 to 15 MHz has been desired. Piezoelectric ceramics of a PZT (lead zirconate titanate) series has been used as a piezoelectric ceramic material of such a piezoelectric resonator of a thickness-extensional vibration mode. The fundamental wave of the piezoelectric ceramics of a PZT series is in the range of 6 to 13 MHz. In order to vibrate the piezoelectric ceramics of a PZT series in a higher frequency band, the thickness of a piezoelectric ceramic body of the piezoelectric resonator must be decreased. However, such piezoelectric ceramics of a PZT series is inferior in heat resistance, temperature characteristics and shock resistance. If a piezoelectric resonator in a high frequency band is mass-produced, therefore, the fraction defective thereof is increased.
Furthermore, it is considered that a piezoelectric resonator in a high frequency band utilizing a third harmonic wave of a thickness-extensional vibration mode is manufactured. As a piezoelectric ceramic material of such a piezoelectric resonator utilizing a third harmonic wave, a material of a lead titanate series is considered. The frequency band of the third harmonic wave of such a piezoelectric ceramic material of a lead titanate series is in the range of 12 to 40 MHz. In order to change the frequency band of the third harmonic wave into a frequency band of 10 to 15 MHz, therefore, the thickness of a piezoelectric ceramic body of the piezoelectric resonator must be increased. Consequently, the damping effect of the piezoelectric resonator is increased and the shape thereof becomes large.
However, a piezoelectric material of a lead titanate series is low in dielectric constant, has relatively large piezoelectric characteristics, and has a high mechanical quality factor Qm. In addition, a piezoelectric material of a lead titanate series has some superior features. For example, it has a high Curie temperature. Further, it is not easily degraded even at high temperatures. Consequently, it is preferable to use such a piezoelectric material of a lead titanate series. In order to utilize a frequency band of 10 to 15 MHz without increasing the thickness of the piezoelectric ceramic body, it is preferable to trap the energy of the fundamental wave of a thickness-extensional vibration mode of the piezoelectric material of a lead titanate series.
Since the Poisson's ratio of the piezoelectric material of a lead titanate series is not more than 1/3, however, it is known that the fundamental wave cannot be trapped even in a general structure. In order to trap the fundamental wave in a piezoelectric material having a Poisson's ratio of not more than 1/3, as shown in FIG. 7, the piezoelectric resonator must have a structure in which the thickness in the center of a piezoelectric ceramic body 1 is decreased and the thickness of a portion interposed between excitation electrodes 2 and 3 is decreased. Alternatively, as shown in FIG. 8, it must have a structure in which another electrode 6 which is not electrically connected to an excitation electrode 2 is provided around the excitation electrode 2 to short-circuit the electrodes on both sides of the ceramic body 1.
Such structures are complicated, so that it is difficult to process to make the structures.